This invention relates to a hybrid electric powertrain, including an electrically variable transmission operable in input-split and compound-split modes.
An electrically variable transmission utilizes one or more electric machines and a differential gearing arrangement to provide a continuously variable ratio drive, and is particularly useful in hybrid electric powertrains including an engine that is operated at a constant speed. Such a powertrain is shown and described in the U.S. Pat. No. 5,931,757 to Schmidt, issued on Aug. 3, 1999, and assigned to the assignee of the present invention. In Schmidt, the differential gearing arrangement provides a compound split mode of operation in which the input power is divided into two power paths through a first differential gearing arrangement and then combined at the output through a second differential gearing arrangement. Operation over a wide output speed range without requiring undesirably high electric machine speeds is achieved by using clutches to reconfigure or shift the gearing arrangement at a mid-range output speed. The principle object of this invention is to provide a hybrid electric powertrain that exhibits the advantages of the Schmidt powertrain, but with reduced mechanical complexity and improved ease of control.
The present invention is directed to an improved hybrid electric powertrain including an electrically variable transmission having two differential gearsets coupled to an engine and first and second electrical machines, wherein the gearsets are configurable in input-split and compound-split modes, and wherein shifting between the input-split and compound-split modes occurs at a zero speed point of one of the electric machines for synchronous clutch engagement. The input-split mode provides reverse and low forward ranges, with the first machine operated as a motor, and the second as a generator. The compound-split mode provides a high forward range, with the machines selectively operated in motoring or generating modes depending on the speed of the vehicle. Shifting between the input-split and compound-split modes occurs synchronously, and at a zero speed of the second machine, contributing to an exceptionally smooth shift. In the illustrated embodiment, the differential gearsets include first and second planetary gearsets having coupled planet carriers, with the transmission output coupled to the planet carriers, the first machine coupled to the sun gear of the second gearset, the second machine coupled to the sun gear of the first gearset, and the engine coupled to the ring gear of the first gearset. A first clutch grounds the ring gear of the second gearset during the input-split mode, and a second clutch couples the ring gear of the second gearset to the sun gear of the first gearset during the compound-split mode.